1. Field of the Invention
This invention relates to cavity length control systems for ring laser gyroscopes, and more particularly, to a cavity or pathlength control which digitally provides adaptive control of the ring laser gyroscope cavity length.
2. Description of Related Art
The planar ring laser gyroscope was a first attempt at a non-mechanical truly strap-down inertial navigation sensor. At low rotation rates, the retroscatter from the mirrors couples energy from one of the oscillating beams into the oppositely propagating beam which locks the oscillating frequencies together yielding zero rotation information at low rotation rates. Current operational ring laser gyroscopes having a planar configuration use mechanical dithering schemes to bias the rate sensor to avoid this well known lock-in phenomenon. Mechanical dither is very effective in reducing the effects of lock-in and makes the ring laser gyroscope a viable navigational gyroscope. However, an effective mechanically dithered ring laser gyroscope adds a noise component to the output of the ring laser which in turn reduces its ultimate accuracy. Also, the presence of mechanical dither, either in the mirrors or full body dither, detracts from the desired goal of a fully strapped down inertial navigational unit.
With these problems in mind, alternative biasing techniques have been developed using the nonreciprocal Faraday effect by either applying a magnetic field to a magnetic mirror (using the Kerr effect) or directly to the gain medium (using the Zeeman effect), or to a solid glass element known as a Faraday rotator, which when used in combination with the magnetic field, provides a Faraday effect phase shift for one beam that is opposite the phase shift of the oppositely directed beam whereby two counter rotating beams are split in frequency. To achieve actual phase shifts instead of simple polarization rotation, two pairs of oppositely directed circularly polarized beams are optimally present within a single optical path to achieve a desired result. An example of this theory of multioscillator ring laser gyroscope may be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued Apr. 4, 1989 to Raytheon Corporation (Terry A. Dorschner, inventor). The nonplanar ray path produced in a multioscillator ring laser gyroscope ensures circular polarized reciprocally split light. The nonplanar ray path reciprocally rotates the polarizations by many degrees yielding the necessary circular polarization. The nonplanar reciprocal phase shift also achieves two Faraday bias gyroscopes, the gain curve 10 of which is illustrated in FIGS. 1A through 1C. FIGS. 1A through 1C show graphical representations of the power gain curve 10 of a multioscillator ring laser gyroscope (and any variations of these gain curves are shown at 10'). The nonplanar ray path splits the light through its geometry into two separate gyroscopes, one being left circularly polarized (LCP) and the other right circularly polarized (RCP). This splitting is known as reciprocal splitting and typically is in the range of 100's of MHz. By placing a Faraday element in the beam path of a nonplanar ring laser gyroscope, when the proper magnetic field is applied to the Faraday element, nonreciprocal splitting of each gyroscope is achieved.
An integral number of wavelengths around the light beam cavity path of the gyroscope is required to support resonant operation. As shown in FIG. 1A, at least four modes are produced: a left circularly polarized anti-clockwise beam (W.sub.LA), having an amplitude (A.sub.LA), a left circularly polarized clockwise beam (W.sub.LC), having an amplitude (A.sub.LC), a right circularly polarized clockwise beam (W.sub.RC), having an amplitude (A.sub.RC), and a right circularly polarized anti-clockwise beam (W.sub.RA), having an amplitude (A.sub.RA). FIG. 1A is a graphical representation of the power gain curve 10 of a multioscillator ring laser gyroscope. An integral number of wavelengths around the light beam cavity path of the inertial system is required to produce oscillation W.sub.LA, W.sub.LC, W.sub.RA, and W.sub.RC. The Faraday (nonreciprocal) splitting between clockwise and anti-clockwise modes is about 1 MHz. At least four mirrors form the ring resonator path, which contains the two gyroscopes, left and right circularly polarized. One of the mirrors is slightly transmissive to allow light to leave the resonator and impinge upon a photo detector for signal processing. When the signals are processed electronically to remove the Faraday bias, the scale factor of the gyroscope is doubled over the conventional ring laser gyroscope. The nonplanar geometry multioscillator ring laser gyroscope using a Faraday element is currently manufactured using a gas discharge pump to provide the active medium, which occupies a portion of the light beam path. The multioscillator ring laser gyroscope produces two signals which are optically biased (due to the Faraday cell). One signal frequency is the Faraday frequency plus one half the rate frequency; the other is the Faraday frequency minus one half the rate frequency. The gyroscope outputs the phase (integrated frequency) of these two signals. Their difference represents a rotation angle increment. However, the output signals are quantized at discrete levels separated by 2.pi. of the gyroscope phase (i.e., an interference fringe).
The two output signals from the multioscillator are produced by heterodyning the like-polarized counterpropagating optical signal beams. Such signals are called heterodyne signals. One or more heterodyne signal is created for the left hand circularly polarized gyroscope and one or more is created for the right hand circularly polarized gyroscope. This can be accomplished either with an optical polarizer or other signal processing scheme. The heterodyne signals represent intensity fringes.
Optical signal fringes are produced by the heterodyne signals and detected by a pair of photo sensors. The curve 10 of FIG. 1A will hereinafter be assumed to be symmetrical about a central axis frequency (W.sub.C), W.sub.C located at the center of maximum gain. Thus, the power amplitudes A.sub.LA and A.sub.LC corresponding to the frequencies W.sub.LA and W.sub.LC are respectively symmetrical to the power amplitudes A.sub.RA and A.sub.RC with respect to the central axis. As will be described later in greater detail, the cavity length control apparatus of the present invention causes the inertial sensor to operate at a substantially maximum power gain, and maintains this maximum gain condition. In this regard, and for the purpose of retaining a maximum overall output power gain for the inertial sensor, the clockwise propagating beams corresponding to the frequencies W.sub.LC and W.sub.RC are mixed and processed. It should be understood, however, that the counter-clockwise propagating beams corresponding to the frequency W.sub.LA and W.sub.RA can be alternatively mixed and processed. However, it has heretofore been known that only two modes are needed to accurately control the cavity length of the inertial sensor.
Heretofore, in a two mode ring laser gyroscope, portions of the counterpropagating beams are superimposed against each other to produce interference fringes which are counted as measures of angular displacement and velocity of the ring laser about a sensing axis. In such gyroscopes, to achieve a consistent calibration of the gyroscope, it is essential that the physical lengths of the paths be maintained. This is also true for multioscilators. To maintain the physical lengths, the ring laser cavity is preferably imbedded in a dimensionally stable laser block. A set of mirrors, ( numbering four or more in non-planar multioscillators) are positioned at the intersection of the bores or cavities defining the optical path and such mirrors may be called, "corner mirrors." For convenience of explanation, the ring laser is described with four mirrors and four bores, cavities, or legs.
A ring laser gyroscope is here described with a ring laser having at least two sets of counterpropagating beams (forming at least two independent gyroscopes) traveling around the laser path in the same physical bore space. At least one of the corner mirrors transmits a very small amount of the counterpropagating laser beams to an optical system which usually uses prisms to collect and superimpose them. A sensor senses the interference fringes produced by the superimposed beams, and electronics apparatus responsive to the detected signals counts the fringes and computes the fringe rate, angular displacement, and angular velocity of the laser about a predetermined axis.
One significant problem in any ring laser gyroscope arises in tuning the ring laser cavity to the correct length to support the resonant modes of the counterpropagating beams. The inward-outward position of at least one of the corner mirrors is adjustable to control the cavity length of the ring laser. Although only one adjustable mirror is needed, the apparatus for controlling cavity length described herein may use two adjustable mirrors which increases the range of adjustment of the cavity length.
The partially transparent corner mirror may be any mirror, but it is preferably not one of the movable mirrors. The beams extracted through the partially transparent mirror produce an optical output signal whose count is a measure of the angular displacement sensed by the instrument and whose amplitude is a measure of laser intensity. If desired, intensity and angular information can both be derived from the signal through a single partially transmissive mirror. Alternatively, two different partially transparent mirrors may be used.
A transducer, preferably a piezoelectric transducer having driving electrodes, forces the movable mirror or mirrors inwardly or outwardly, and the amount of inward or outward motion depends upon the voltage delivered to the electrodes.
The scale factor between the amount of voltage applied to the transducer electrodes and the excursion of travel of the mirror attached to the transducer, varies with many factors including but not limited to temperature of the mirror and the transducer, the compliance of the flexure springs supporting the transducer, and the bonding of those flexures. As the transducer scale factor varies, the ratio of its applied control voltage to the corresponding excursion of its attached movable mirror varies, and the amount of voltage change to move the movable mirror inwardly and outwardly to change the cavity length by one laser beam wavelength also varies.
Heretofore, a computer, usually the system computer used for the ring laser, generates digital words or bytes, converts them into an analog signal, and delivers them to control the inward and outward position of the piezoelectric transducer and its attached movable corner mirror. The lasing intensity peaks at inward-outward positions of the movable corner mirror corresponding to cavity lengths that are separated by a distance of one wavelength of the laser beam.
Cavity length control (in a two mode gyroscope) historically was achieved using a "hill climbing" servo which employed analog modulation of the mirror transducer drive voltage followed by analog demodulation of the intensity signal. The modulation/demodulation took place at a fairly high frequency (e.g. six kilohertz). The servo could then be closed via an analog loop which fed back a control voltage which was dependent on the output of the demodulator. A stable operating condition was achieved when the demodulator output was zero on average.
Later, the servo operations were performed by the system computer. An analog-to-digital converter was used to allow the computer to command the control voltage. The apparatus still relied upon the basic six kilohertz (or equivalent) analog modulation and demodulation to produce an error signal for operation of the control loop.
A study revealed that, because of variations in the sensitivity of piezoelectric transducers and of other mirror and gyroscope parameters, such servo loops exhibited very large loop gain variations, thereby leading to inconsistent controller performance and often long convergence times. To solve this problem, two of the applicants hereto, have filed a co-pending patent application, (assigned to the common assignee of this application) entitled "COMPUTER GENERATED CAVITY LENGTH CONTROL WITH AUTOMATIC GAIN CONTROL FOR RING LASER GYROS" on Nov. 1, 1991, Ser. No. 07/640,179. In this co-pending application a primary ("hill-climbing" servo-loop) and secondary (modulation control) servo-loop provide fine tuned control of the cavity length for a two mode ring laser gyroscope. In particular, with reference to FIG. 1B, the 07/640,179 case taught the need to adjust modulation control over a widely varying temperature range, thereby controlling the temperature sensitivity of the cavity length of the ring laser gyroscope. This second harmonic demodulation allowed the cavity length control system to accommodate gain curves like 10' (FIG. 1B) which are characterized by PZT (piezo-transducer) gain changes. By controlling modulation gain and depth, one can accommodate the changing curvature of the intensity gain curve 10' of FIG. 1B over a widely varying temperature range.
Heretofore, the applicant's assignee is also owner of an issued U.S. Pat. No. 4,963,026 (granted Oct. 16, 1990) entitled "CAVITY LENGTH CONTROL APPARATUS FOR A MULTIOSCILLATOR" which teaches an analog primary "hill-climbing" servo-loop subjected to an RF amplitude modulated signal in the range of 1-10 KHz. It is the applicant's desire to take the teaching from the co-pending 07/640,179 case and apply it to the multioscillator situation contemplated in the 4,963,026 patent and improve upon it.